Radiofrequency (RF) thermal ablation has been employed for a number of years as a means of modifying neural tissues, typically as a treatment for pain. The RF signal is delivered using a percutaneous needle with an insulated shaft and exposed tip which is positioned over the target nerves. Temperatures of minimum 60° C. are used to produce long-term pain relief through coagulation of tissue. Control of probe tip temperature is important; typically a temperature sensor at the tip is used so that power delivery can be modulated appropriately. RF devices deliver energy in the form of a sinusoidal waveform at 250 to 500 KHz, well above the range at which nerve fibres will respond. At the frequencies concerned, tissue impedance is typically in the range of 100 to 500 ohm and the peak voltage of the waveform is 60 to 100 V.
Pulsed radiofrequency (pulsed RF) has emerged in recent years. Pulsed RF uses signals which are interrupted so that tissues surrounding the needle tip have time to dissipate heat and the temperature does not exceed 42-43 degrees centigrade, below the point at which tissues are damaged. Pulsed RF is described by Sluijter et al “Method and apparatus for altering neural tissue function” U.S. Pat. No. 5,983,141 and U.S. Pat. No. 6,259,952. Pulsed RF is considered by many to be a safer alternative to RF ablation as the lack of heat generation limits damage to nearby structures.
The effect of pulsed RF and to a lesser extent thermal RF is temporary because the nerves regenerate, typically lasting for a few weeks to several months after which an additional treatment may be required.
Targets for pulsed RF include facet joint arthropathy, failed back surgery syndrome, nerve root compression, neuropathic spinal pain and chronic headache. For example, pulsed RF applied to the occipital nerve using a percutaneous needle as a treatment for cervicogenic headache may prevent attacks for a number of weeks.
In recent years thermal RF has been employed for treatments other than pain control. An example is renal nerve ablation for modification of sympathetic activity as a treatment for hypertension, described by Demarais et al “Methods and apparatus for thermally-induced renal neuromodulation” U.S. Pat. No. 7,617,005. Around one third of the adult population has high blood pressure or essential hypertension, many patients need a combination of 2 or 3 antihypertensive drugs of different classes to adequately lower their blood pressure, but in a significant minority even such a combination of drugs is ineffective. Presently there are only a limited range of treatment options available for patients who develop drug resistance.
In response to this unmet need, other targets and methods of modulating blood pressure by electrical stimulation have been developed. Mayberg “Brainstem and cerebellar modulation of cardiovascular response and disease” WO2004069328 describes a method of control of blood pressure employing electrodes in the brainstem. Kieval et al. “Baroreflex modulation to gradually decrease blood pressure” U.S. Pat. No. 8,060,206 B2 describes simulating baroreceptors in the region of the Carotid sinus to control blood pressure.
In order to facilitate delivery of electrical stimulation chronically, or to locations that are difficult to reach with percutaneous needles, various devices that are implantable but employ external power sources have been developed. Gleason etc al “Implantable nerve stimulation device” U.S. Pat. No. 5,094,242 describes an implantable neurostimulation device which receives energy from an external coil via induction and delivers this energy to an implantable receiving coil. Such inductively coupled devices are well suited to neurostimulation applications as the power requirements of such devices are in the region of a few milliwatts. For RF or pulsed RF devices, the peak power requirements during the pulse is typically in the range of 20 W or more, which requires a powerful external coil as transmission efficiency is unlikely to exceed 10%.
Means of delivering electrical energy by direct electrical connection via a percutaneous needle to an implantable device are also described in the prior art. For example, Mantsch el al “Medical Electrode System” US2008/0215126, now U.S. Pat. No. 9,042,998, describe a flexible electrode and catheter connected to an implantable port into which an external needle is introduced percutaneously for delivery of RF and/or drugs into the epidural space. Malaney et al “Implantable electro-acupuncture device” U.S. Pat. No. 6,377,853 describe a device which is intended for nerve stimulation also employing a needle and implantable port. In both examples, a cone is employed to guide the needle into a contact port so that errors in alignment of the implantable connector and percutaneous needle can be compensated.